1. Field of the Invention
This invention relates to heat pump systems for selectively heating and cooling an environment or enclosure housing at least one heat exchange coil of the heat pump system, while rejecting heat or adding heat thereto by way of a second coil external of the enclosure and subject to ambient, and more particularly, to the employment of a multiple slide valve helical screw compressor within such heat pump system for improved efficiency and low operating costs.
2. Description of the Prior Art
With fossil fuel reserves diminishing rapidly, it is inevitable that this country and the world will shift more and more to central station electric power generating facilities. One of the major practical solutions to the heating and cooling requirements of this nation is the utilization of an extremely efficient, reliable and reasonably priced electrically driven heat pump. A heat pump, by its very nature, comprises a reversible closed loop refrigeration system in which a compressor within the loop compresses a gaseous refrigerant from low pressure to high pressure, a first coil downstream of the compressor condenses the gaseous high pressure refrigerant to a liquid and an expansion valve between the first coil and a second coil permits the high pressure liquid refrigerant to expand within the second and downstream coil for cooling the environment within which that coil is placed by way of the latent heat of vaporization of the refrigerant, with the refrigerant vapor returning through the closed loop to the compressor for recompression. Conventionally, such a compressor is driven in a single direction and in order to effect reverse heat pump operation wherein the first coil absorbs heat from the environment and the second coil rejects heat to effect condensation of the compressed refrigerant gas, a reversing valve is provided to connect the discharge of the compressor to the other of the two coils and the suction to the coil previously connected to the discharge.
Within recent years, the helical screw rotary compressor has come into vogue, the helical screw rotary compressor being an inherently reliable type machine having a volumetric efficiency which is characteristically best suited for heat pump service. In contrast to the typical reciprocating compressor, wherein the volumetric efficiency of the compressor deteriorates rapidly as the pressure ratio imposed upon it by the system increases, there is no such rapid deterioration in volumetric efficiency with a screw compressor. Thus, the screw compressor provides an ideal match for heat pump requirements in that as the ambient temperature falls during the heating season, the CFM pumped by the compressor does not deteriorate as would occur by a conventional, single stage reciprocating compressor.
Applicant in his prior application Ser. No. 492,084 entitled "Undercompression and Overcompression Free Helical Screw Rotary Compressor" filed July 26, 1974, and now U.S. Pat. No. 3,936,239 provides within such helical screw rotary compressor a slide valve member which controls the discharge pressure of the compressor and which includes a port opening to a closed thread adjacent to the end of the slide valve member closing off the discharge port to the closed thread for sensing that closed thread pressure and the helical screw rotary compressor further comprises means for controlling the shifting of that slide valve member to equalize these pressures and to thus prevent undercompression or overcompression of the compressor working fluid within the closed thread prior to discharge. The helical screw rotary compressor may be of the reversible type and may employ a second identically formed, axially shiftable slide valve member with the dual slide valve members interchangeably performing functions of compressor capacity control and prevention of undercompression or overcompression of the compressor.
In refrigeration and air conditioning systems, it is conventional to bleed a portion of the liquid, high pressure refrigerant downstream of the system condenser and expand that liquid refrigerant in a heat exchange coil operatively positioned with respect to the refrigerant line leading from the condenser to one or more of the evaporator coils for subcooling the condensed high pressure refrigerant prior to employing its energy content in cooling the evaporative load. Further, it is conventional to employ multiple evaporators tailored to the diverse cooling loads, in which case the vaporized refrigerant leaving the evaporator coils of the various evaporators and returning to the compressor are at different pressures.
It is therefore an object of the present invention to provide an improved heat pump refrigeration and heating system which employs a helical screw rotary compressor which will operate on either a heating or a cooling cycle with wide variation in ambient conditions and wide variations in compressor loading with no loss in efficiency.
It is therefore a further object of the present invention to provide a helical screw rotary compressor within a heat pump heating and cooling system which is characterized by a variable built in pressure ratio with the compressor automatically and completely adjusting to pressure conditions and loading conditions imposed on it by the refrigeration system.
A further object of the present invention is to provide an improved heat pump heating and cooling system which employs a helical screw rotary compressor which matches compressor discharge to line pressure, and wherein the return flow of refrigerant vapor from the subcooling or economizer coil or an intermediate pressure evaporator coil may be injected into a helical screw compressor closed thread intermediate of the suction and discharge ports of the compressor.
It is a further object of this invention to provide a helical screw compressor for use in a heat pump heating and cooling system wherein the compressor employs multiple, axially shiftable slide valves for: (1) controlling the capacity of the compressor; (2) matching the closed thread pressure of the compressor at discharge to the discharge line pressure; (3) controlling the point of injection of a refrigerant gas return from a subcooling or economizer coil or a high pressure evaporator coil depending upon system conditions; and (4) axially adjusting the point of working fluid vapor removal and return to compressor closed threads feeding a secondary closed refrigeration loop for subcooling the main loop refrigerant liquid or other function.